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Christopher W. Tyler; The null-contrast necker cube: A geometric depth stimulus invisible to known cortical mechanisms. Journal of Vision 2002;2(7):675. doi: 10.1167/2.7.675.
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Purpose. An interesting type of probe stimulus is one that is designed to be null with respect to the responses of known classes of cortical neurons. If such a stimulus is visible, the perceptual processing must be carried by undiscovered types of neural processing. Methods. The classic Necker cube may be generated in the novel form of a null-contrast image, gray bars against a background of high-contrast random texture of the same mean luminance as the bars. Regions of zero contrast produce no response in all known cortical neurons, so the brain has no obvious mechanism to identify the gray regions in the image. In particular, there is no signal to be integrated along the length of the bars and support their unification into a perceptual whole. Plausible computational models of simple, complex and hypercomplex neurons were developed on the basis of established neural models. Simulations revealed that none of the neuronal types responded coherently to the presence of the gray bars, being activated only by the noise background. Results. Statistical evaluation of the simulated responses revealed no trace of the gray bars that could be detected by any signal operator (such as a peak detector) because the ‘bars’ only exist as an increased number of zeros among the background signals. Nevertheless, perceptual observation of the null-contrast stimulus revealed that it was subject to the well-known depth reversals of the standard Necker cube. These observations imply not only perceptual integration into a unitary figure, but one that can be interpreted three-dimensionally. Conclusion. This new class of null stimuli therefore represents a challenge for visual neuroscience, to explain the observed spatial integration of regions of no neural signal into coherent line structures supporting 3D percepts.
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